For adhesives, in particular structural adhesives which are used in precision mechanics and optics, there is an increasing need for short curing times. Reaction adhesives with short curing times usually also have short processing times. However, from a technical point of view sufficiently long processing times are often required, for example in order to align precisely the work pieces to be bonded. Polyurethane adhesives and amine-cured epoxy resins are known as commercially available adhesives which cure at room temperature. With a processing time of approx. one hour, the curing time at room temperature to achieve the final strength is in the range of around one to two days.
A reduction in the curing time can be achieved by using light-curing adhesives, provided that the adherend surfaces are sufficiently translucent and there are no shadow zones. In the case of large adhesive surfaces it is necessary to realize a homogeneous illumination level so that the adhesive can cure uniformly and with low stress. In addition, hybrid systems or dual-curing adhesives are known as alternatives which, in addition to UV curing can also cure in a darkness curing through humidity or through heat. The investigations on commercially available dual-curing structural adhesives carried out within the framework of the present invention have shown, however, that the polymer formed by darkness curing differs significantly from the polymer formed by UV polymerization. Without exception, the usage properties of investigated cured adhesives were not satisfactory. Furthermore, the conditions required for a dark reaction are unfavourable for bonding precision mechanical and optical devices since the humidity in cleanrooms often falls below the required humidity of at least 50% relative humidity and, in addition, optic housings are often flushed with nitrogen for drying.
For thermal post-curing reactions, temperatures of 80° C. and above are often required, which can lead to stresses or sometimes to damage to components. In the case of anaerobic darkness curing, the problem arises that the materials suitable for this are often not present in precision mechanical optical devices and special primers must be applied as activators before the adhesive process.
In the case of commercially available rapid-curing adhesives based on epoxides, despite processing times of often only a few minutes, the final strengths are only achieved after approx. 5-20 hours (in the case of curing at room temperature). In the case of such short processing times, there is also the problem that the wetting of the adhesive surfaces rapidly becomes insufficient, whereby the bonding strength deteriorates significantly, in particular after exposure to damp heat.
For the microscopic investigation of samples, for example tissue sections, it is usual to embed the samples in a transparent medium. The embedding medium creates the optical conditions for the microscopic investigation, protects the sample from mechanical damage and serves to preserve the sample in the long term. For high image quality, the optical properties of the embedding medium are of decisive importance, in particular a refractive index and dispersion which can be adapted to the microscopy technique used and the type of preparation to be investigated as well as a high transmittance and low residual fluorescence of the embedding medium. The refractive index of the embedding medium is to be able to be adapted as closely as possible to the refractive index of the glass of the specimen slide used and cover glass or, in the case of immersion microscopy, to the refractive index of the immersion medium, in order to achieve as low a spherical aberration as possible. In the case of such embedding media, a short curing time accompanied by a sufficiently long processing time, curing at room temperature to protect the biological samples to be embedded and good adhesion to the embedded samples are also desirable. In the case of specific preparations, however, it can also be necessary for the (cured) embedding medium to have as high a refractive index as possible, whereby high imaging qualities are then produced during microscopic examination.
Furthermore, in the case of specific optical applications, it can be necessary for the refractive power in the cured state of an adhesive used to be adapted to the refractive power of the optical components used. Adhesives for optical components are also referred to as cements. For example, in prism groups which are used as beam splitters, the wedge error unavoidably produced by the cement layer on cementing two prisms can be eliminated by adapting the refractive power. It is thereby possible to improve the imaging quality of the prism group considerably.
Accordingly, there is a need in the industry of adhesives for microscopy and optical elements that address the problems associated with the prior art while maintaining or even improving the high image quality.